From the perspective of surface tension, if the ink is required to quickly detach from the printing plate, so that the ink does not remain on the printing plate, of course, the surface tension of the printing plate and the surface tension of the ink should be similar in value to make the ink on the printing plate. The wetting angle is close to 90 °. However, because the printing plate in the flexographic process is responsible for the transfer of ink, it is necessary to first draw ink from the anilox roller, and then transfer it to the printing material as little as possible, so the printing plate and It is unrealistic that the surface tension of the ink is absolutely equal. A reasonable choice is that the surface tension of the printing plate is slightly greater than the surface tension of the ink, but the value needs to be as close as possible. Only in this way can the printing plate not only maintain good inking properties, but also achieve good ink transferability.
When flexo printing is blocked, the accumulated ink on the printing plate is not accumulated in the inked area, but the adhesion in the depressed area. It should not be absolutely equal. According to the analysis of most flexographic printing plates now, there is no difference between the inked area and the depressed area of ​​the plate itself. The polymer materials constituting the plate are the same. If there is no interference from various plate washing solutions in the plate washing process, The surface tension should also be the same.
However, the test data shows that the surface tension of the inked area of ​​most printing plates is lower than that of the depressed area. This is because: after the main exposure process of UVA exposure, the strength of the hardened resin and the uncured resin are very different. The unphotosensitive hardened resin is washed away under the action of the plate washing brush, and the brush is left on its surface Obvious traces; the photo-hardened resin has high strength, which not only resists the erosion of the plate washing solution, but also avoids the damage to the surface of the brush. It can be seen under a high magnification magnifying glass that the inked area of ​​the printing plate is smooth like a mirror, and the layout is smooth, but the marks of the depressed area cleared by the brush are obvious and slightly rough. Therefore, the surface tension of the depressed area of ​​the printing plate is higher than the inked area by 1-2 mN / m.
But the actual demand is: high surface tension in the inked area of ​​the printing plate and low surface tension in the depressed area. The surface tension of the inked area is high, the printing plate has good inking performance, and the amount of ink drawn from the anilox roller is large; the surface tension of the depressed area is low, and the ink overflowing from the surface of the dots cannot stick to the depressed area and cannot be in the printing plate Accumulation of ink can effectively avoid plate jamming failure.
The effective use of the characteristics that the surface tension of the liquid will decrease as the temperature rises, and that the surface tension of the solid will increase as the surface shear strengthens, may be an effective way to find the appropriate surface tension of the printing plate. In this way, it is possible to have such a printing plate: the depressed area of ​​the printing plate presents a low surface tension during the printing process, so that the ink is not easily contaminated on it; the surface tension is increased by the shearing of the ink plate and the anilox roller, The inking performance of the printing plate is good. The final confirmation of the surface tension of the printing plate must match the surface tension of the ink used, which requires careful calculation and demonstration.
In order to solve the hidden danger of flexo blocking from the aspect of printing plate quality, a printing plate with suitable surface tension and stable numerical value is needed. This is the new demand of the printer for the quality and performance of the printing plate.
When the plate-making process can provide a plate with stable surface tension for the flexo process, the next problem is ink. Our goal is to need an ink that effectively matches the surface tension of the printing plate, whether it is solvent-based ink, water-based ink, or UV ink.
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